Three-Winding Transformers

Three identical single-phase transformers form a three-phase connection with high-voltage and low-voltage terminals configured across all four winding types.

Base quantities for a grounded Y-Y transformer's per-unit equivalent circuit include a common base complex power and a voltage base ratio matching the ratio of rated line-to-line voltages.

Balanced currents in this transformer yield zero neutral currents, making its per-unit equivalent circuit similar to a single-phase ideal transformer.

Extending two-winding transformer equations gives the relations needed for a single-phase three-winding transformer.

These relations, written in actual and per-unit measures, satisfy the per-unit equivalent circuit.

The practical three-winding transformer circuit includes external series impedance and shunt admittance branches evaluated via an open-circuit test.

With one winding open, the three-winding transformer acts like a two-winding one. Standard short-circuit tests evaluate per-unit leakage impedances, further simplified by neglecting the shunt admittance branches.

Finally, per-unit series impedances are derived from the leakage impedances.